The detection of so-called “suicide bombers”, who carry bombs on their body and activate them immediately at the slightest sign of a security response, continues to be a largely important security issue today.
At present, metal detectors are used, as well as various types of gas trace detectors, X-ray machines and specially-trained dogs. Currently, developed detection approaches in various countries include detectors that are based on the following principles: NQR effect, Raman backscattering, dielectric portals, passive and active devices for human body inspection (terahertz frequencies), passive radars (millimeter frequencies) and active microwave portals.
Modern inspection methods and devices continue to lack important features: they don't provide standoff covert inspection (i.e., they cannot detect a “suicide bomber” in real time or perform counteractions against him before he activates an explosive); they cannot automatically determine the danger/risk level of the detected object and have a very high false alarm rate, which limits use in real conditions, e.g., in a moving crowd.
The prior art, in general, lacks at least half of the following features: Standoff inspection; automatic inspection; real time inspection; covert inspection; environmental independence; safety for human health; possibility to associate an alarm signal with a certain person; mobility; and relatively low cost.
The present invention provides for a method for standoff detection of objects based on measuring a thickness of said object and further calculating a dielectric permittivity value; comparing said dielectric permittivity value to a database of reference dielectric permittivity values, so as to determine to which preselected group of objects the object belongs and whether the object belongs to a preselected group of dangerous objects. Goods stolen from a supermarket can e.g. form a preselected group of objects. A preselected group of dangerous objects could in particular be formed by a group of explosive materials or a group of improvised explosive devices (IED).
A variety of methods exist for measuring a complex dielectric permittivity value of solid materials using high frequency techniques. Methods employing microwave frequency ranges are based on electromagnetic wave propagation in a medium or wave processes on the borders of two media. All known methods are based on registration of the phase change when the microwave passes through the dielectric object. These methods operate by linking the phase change value to the value of the dielectric permittivity of the target material. These connections can vary in each particular case, thus explaining the variety of measurement methods used to determine a material's complex dielectric constant (∈∈=∈′+∈″) and dissipation factor (tan(δ)=∈″/∈′), wherein ∈′ and ∈″ are the real and imaginary parts of the dielectric constant, respectively.
Several methods exist for measuring the dielectric constant of material based on the analysis of a signal at high or super-high frequencies:
(1) Methods which use directional waves: waveguide methods with coaxial line and rectangular waveguides; using one of the most common waveguide methods—the short-circuit method—one determines characteristics of a dielectric sample located at the shorter end of the waveguide according to the phase and coefficient of a wave moving along the line.
(2) Resonant methods, which measure resonance frequencies and quality factors.
(3) Methods which use waves in free space, e.g., based on measuring the coefficients of reflection and transmission, i.e., quasi-optic methods used to measure parameters in free space.
The choice of a method to be used is determined by the type of measurements (laboratory researches, industrial nondestructive control, etc.), frequency range, and a material's characteristics. The main disadvantage of the second and third methods above (2, 3) is their incompatibility with odd or abnormally shaped objects. Such methods are capable of producing measured samples of materials having two plane surfaces (e.g., rectangular). Due to the variety of shapes of dangerous dielectric objects today, existing methods must be improved to carry out standoff inspection of a monitored space and determine the dielectric characteristics of all objects, including those that are irregularly-shaped.
Additionally, methods using directional waves (1, above) and resonant methods (2, above) are the most accurate methods in the decimeter and centimeter wave range; however, they require using a samples fitting waveguide or a resonator line cross-section.
The closest prior art to the proposed method is a method to measure a dielectric constant described in RF Patent No. 2418269, “Method and device for tomographic measurements of multi-phase flow.” This disclosed method is based on the irradiation of a dielectric multi-phase liquid medium (gas-liquid mixture), located inside a Venturi tube, with microwave electromagnetic radiation, further comprising recording and analysis of the transmitted field. The complex dielectric constant is determined by measuring the phase constant and the attenuation rate of a plane electromagnetic wave propagating inside the Venturi tube. The method comprises measuring the difference between phases of electromagnetic waves for two receiving antennas, placed within the tube at different distances from a third, transmitting antenna. The phase is measured at two or more frequencies, within the range of 1 MHz and 10 GHz. The attenuation rate is measured similar to the phase constant of the propagating wave, except that, instead of phase difference, the dissipation difference, k=α+iβ (where k=complex propagation constant, α=attenuation rate, and β=phase constant of the wave) is estimated.
The disadvantages of the above method include the following: (1) the requirement to use at least 3 antennas (1 transmitting and 2 receiving antennas); (2) the requirement to use a dielectric liquid in a special Venturi tube, thus not allowing for measurements of solid objects or covert standoff inspection and detection; (3) the receiving antenna is located close to the transmitting antenna, thus the model of plane wave propagation must be corrected considering (a) dependence between the distance between receiving antennas and the length of the wave received by them, and (b) the weak dependence between this distance and the conductivity of the required multi-phase liquid medium (additional dependencies into the algorithm makes required calculations more complex and time-consuming); (4) the method is only useful under laboratory conditions (e.g., detection of planar/simple objects).